1. Field of the Invention
The present invention relates to a composition and method for diffusion coating ceramic materials, and more particularly to a method for diffusion coating ceramic materials using a pulverous coating composition including chromium.
2. Description of Related Art
Ceramic materials are widely used in different branches of U.S. industry. Among their uses are as fire resistant materials in the steel industry and as abrasives in the automotive industry. Tool ceramics are used in the production of glass. Ceramics are also used in construction, electronics and the nuclear energy industry.
Even metals such as titanium and alloy steels tend to have low resistances to certain aggressively corrosive environments such as concentrated chloridic or salt solutions. For example, the working wheels of pumps formed from metals such as titanium often dissolve within a month when exposed to a concentrated sodium hydroxide solution. Ceramic materials tend to be cheaper than metals and are characterized by good chemical resistance in various environments. As a result, ceramic materials are useful for forming protective coatings for chemical storage containers and in the engineering of chemical equipment. Despite these advantages, the brittleness of conventional ceramic materials limits their use in the chemical industry.
Conventional techniques for protecting the surfaces of ceramic workpieces include metallization, flame-spraying, plasma and plasma-layer spraying. Ceramic workpieces treated by these methods remain brittle because these methods affect only the strength and chemical properties of the surfaces of the workpieces and do not affect their internal structures.
Diffusion coating is carried out by heating a workpiece in the presence of a powdered coating composition. The process has the advantages of uniform heating as the process takes place in a powder mixture with low heat conductivity and of deep or complete penetration of components of the powdered composition into the internal structure of the base material. The process strengthens not only the surface of the workpiece but also its structure. As a result, the workpiece obtains a number of physical and chemical properties which decrease its brittleness and improve its resistance to aggressive chemical environments.
Various compositions have been proposed for use in diffusion coating. One proposed composition for use in diffusion coating metal workpieces has the following components:
Chromium 50 wt % PA1 Aluminum Oxide 43-45 wt % PA1 Ammonium Chloride 5-7 wt %. PA1 Chromium 45-50 wt % PA1 Silicon Carbide 3.5-5.0 wt % PA1 Vanadium Boride 0.5-1.5 wt % PA1 Ammonium Halide 5.0-6.0 wt % PA1 Aluminum Oxide Remainder,
This composition provides some surface protection due to the formation of a solid solution of chromium in iron on the surface. The resulting coating has good chemical resistance in corrosive environments of low and medium aggressiveness. As the chromium metal in the coating dissolves in these environments, however, the chemical resistance of the workpiece decreases.
There remains a need in the art for a diffusion coating method and composition for reducing the brittleness of ceramic base materials and for promoting chemical resistance to aggressive chemical environments.